In yeast Saccharomyces cerevisiae, a wasteful futile urea cycle is prevented by the formation of a multienzyme complex between anabolic ornithine transcarbamoylase, OTCase, and catabolic arginase. In the complex, OTCase is inhibited and the two pathways are uncoupled. The regulation of OTCase results from quaternary interactions between this enzyme and arginase in the complex mediated through a new class of intersubunit bonding domains. The broad goal of my research is the characterization from several perspectives, of the sequence of events in the acquisition of quaternary structure in an oligomer and its use in modulating the activity of a component enzyme. The research discussed here is (1) directed at delineating the events responsible for the creation of bonding domains in OTCase, presumably effector binding promoted conformational changes. These will be monitored by several spectral and hydrodynamic techniques. (2) Subsequently, the thermodynamics of association will be determined from the temperature dependence of the association constant between the free enzymes and the complex. Required for this determination is a precise knowledge of the molecular weights and the stoichiometries of the two enzymes in the complex. These three determinations will be done in the analytical ultracentrifuge, and are directed towards determining the forces that stabilize these new intersubunit bonding domains. (3) A requirement of these two experiments is substrate amounts of the two enzymes. The isolation of these proteins will be facilitated by use of stable yeast plasmid enzyme overproducing strains and affinity chromatography.